strangethursday Posted April 3, 2007 Share Posted April 3, 2007 1st is it really possible to have 0psi on a cylinder? Ive dont a ton of compression tests on various engines over the years & the lowest if ever seen is about 30 & that was on a car that wouldnt stay running any longer. My L28 with an N42 head (unsure of block) is reading 125 & 130 on every cylinder but #4 & #6. 2nd i forget how to translate psi to what the compression ratio is. Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted April 3, 2007 Administrators Share Posted April 3, 2007 Yes, it is possible to have 0 PSI cranking compression Lets say a piston rod broke so the piston doesn’t move up and down the cylinder, or a valve is stuck open so the cylinder isn’t sealed for the piston to build any pressure, etc. If you are referring to using cranking compression to calculate static compression ratio. That is virtually impossible! It is though an indicator of dynamic compression ratio, but in no way can it be used to figure out static compression ratio. Too many variables such as cylinder leakage via the valves and rings as the piston travels up the bore, and of course the real biggee is that the intake valve doesn’t fully close till the piston is some point up the compression stroke, (after BDC). Hope that helps, Quote Link to comment Share on other sites More sharing options...
strangethursday Posted April 3, 2007 Author Share Posted April 3, 2007 Thanks. I was just trying to get an idea of what it might be. 1 of these days I'll find out which block I have Quote Link to comment Share on other sites More sharing options...
strangethursday Posted April 4, 2007 Author Share Posted April 4, 2007 Unless by some freak incident where the piston & rod have detached from the crank near the top of cylinder, I think it's the valves. There's no noise that would suggest a problem like that. How can I go about unsticking a valve without rebuiling the head? Seafoam & an oil change? Quote Link to comment Share on other sites More sharing options...
Garrett76Zt Posted April 4, 2007 Share Posted April 4, 2007 If you are referring to using cranking compression to calculate static compression ratio. That is virtually impossible! It is though an indicator of dynamic compression ratio, but in no way can it be used to figure out static compression ratio. It must be some indication though correct? Even if not exact? I would assume that the higher static compression ratio, a general rule would be that your number measured in a compression test would be higher. Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted April 4, 2007 Administrators Share Posted April 4, 2007 Unless by some freak incident where the piston & rod have detached from the crank near the top of cylinder, I think it's the valves. There's no noise that would suggest a problem like that. How can I go about unsticking a valve without rebuiling the head? Seafoam & an oil change? To verify a stuck open valve, pull the valve cover, and rotate the engine through at least 2 complete crankshaft revolutions. While you are rotating the crank, visually verify that all the valves of the affected cylinders are indeed opening up with cam and then closing as well. You may notice just by looking at them, that one of the valves is will appear to be somewhat open when the cam lobe is not in contact with the valve, that will be dead giveaway. If it is indeed a stuck valve, it could be a bent valve due to contact with a piston due to a broken valve spring or some other reason. If that is the case, the head will have to come apart for repair. If it is just a sticky valve and not a bent valve, more than likely it is due to rust between the valve stem and valve guide. Being as I am a perfectionist and if there is ANY reason why any internal part of an engine isn’t functioning even remotely as it should, I always stress that the engine or at lest the offending component should come apart for thorough investigation as to WHY, then make sure measures are taken to prevent that WHY from happening again, and the engine or the offended component, properly machined/rebuilt before returning it to service. Personally, that is the only way I would personally handle this situation for any of my customers or my own personal projects. For those among you that are into the quick and dirty no initial cost fix, (consider this only temporary), if it is a stuck valve, you can remove the rocker and lash pad of the offending valve. Then be absolutely sure that the piston for that valve is at BDC. Now using a soft blunt drift, such as brass or aluminum, take a hammer and tap on the end of the valve stem, (be sure you are perfectly straight up and down to the valve stem so not to impose too much side loading on the valve stem and guide). The valve should bounce and hopefully come to the closed position. Then verify that it doesn’t stick open again during crank shaft rotation. If it continues to stick further and further open, then just remove the head and fix it properly, i.e. take it to a competent machinist, which generally isn’t the cheapest one in the phone book, unless it s friend pulling a favor for you. Good luck, Paul Quote Link to comment Share on other sites More sharing options...
strangethursday Posted April 4, 2007 Author Share Posted April 4, 2007 I know I just haven't even started rebuilding the other 2 motors I have. 1 for this car & 1 for a 75 280. It's probably just wishfull thinking but, this is my daily unfortunately & I DON'T wanna drive my G20 right now! HAHA! Thanks again Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted April 4, 2007 Administrators Share Posted April 4, 2007 It must be some indication though correct? Even if not exact? I would assume that the higher static compression ratio, a general rule would be that your number measured in a compression test would be higher. Garrett, Uhh.. well…. This is bit of a gray area. I choose to error on the conservative side and blanket statement this by just saying no. In general, what you are saying is true. For a particular engine type and displacement, all else being equal, cam, displacement, etc, and IF the two engines had similar leakage rates with the piston throughout its travel in the cylinder, (not just from a leak down test which only measures leakage with piston at the top of the bore), then yes, a higher cranking compression would indicate a higher compression ratio engine. Also keep in mind, the faster you crank the engine, the cranking compression will most likely be higher as there is less time for “leakage” to occur during the cranking cycle. (the Nisan gear reduction starters spin the engine slower than the older heavy bulky starters, so just swapping starters will result in different cranking compression numbers)… i.e. If you slowly hand crank a tightly sealed engine over by hand, you may never build up even 75 PSI, but if you install a good fast starter on it, hold open the throttle valve open, and attach a fully charged strong battery, you could easily hit 180-200 PSI cranking compression. A big cam on a high compression engine should have comparable cranking compression to a small cammed low compression engine of the same displacement, (this is if the compression ratio matches the cam specs that is.) In short, there are too many variables that would take way more time to isolate, verify and account for within any particular cylinder of any particular engine than it would be just verify casting numbers, remove the head, verify bore, stroke, piston dish, and cylinder head volume and calculate it out exactly. The room for error has just been removed, so to speak, by calculating the volume, not allowing leakages to skew the math… . (if that made any sense?...) Quote Link to comment Share on other sites More sharing options...
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